The shaft voltages occurring in electric machines, especially turbosets, are a potential danger for numerous components of the shaft assembly. If uncontrolled electric circuits develop, the parts lying in these paths could be damaged by the effect of current and by spark erosion. The consequences of damage from shaft voltages were able to be safely eliminated by two generally known measures:
Barriers were made by inserting insulating distances so that the direct-axis voltage of the shaft cannot drive any damaging current over the endangered parts. As a rule these insulating distances, especially in turbosets, are placed only on the nondrive end (NDE) of the electrical machine (generator). A consistent insulation of all potentially endangered parts of a steam turbine or those of motor-driven machines would hardly be feasible.
By grounding the shaft on the noninsulated drive end (DE) with grounding brushes so that the shaft is solidly connected to ground potential.
With the introduction of static excitation systems and static cascade connections, a new source of shaft voltage with steep voltage peaks has been added. These steep voltage peaks so greatly impair the grounding brushes, which are operating under less favorable conditions, due to the high surface speeds, and which are on the drive end (DE) of the dynamoelectric machine, that they become ineffective in relatively short time. The steep and high voltage peaks then lead to capacitive displacement currents on the insulating distances of the machine components and to the breakdown of lubricating oil films. The current pulses triggered by the voltage peaks can cause a spark erosion in the bearings of the shaft assembly.
To remedy such difficulties, it is suggested in DE-OS No. 35 11 755 that the "ohmic" grounding brushes placed on the drive end of the electric machine be relieved by providing, on the nondrive end, on which the bearings are galvanically separated from ground, frame and foundation by insulating distances, a current path comprising a contact device with at least one sliding contact and a capacitor connected to it in series between the sliding contact of the shaft and the machine frame or ground.
However, since safe ohmic grounding of the shaft on the drive end of the shaft assembly cannot be attained due to unfavorable contact relationships in conventional machines, and additionally due to inaccessibility in nuclear power plant machines, the discharge of static charges and capacitively coupled voltages can no longer be guaranteed.